1. Technical Field
The present invention relates to polymers prepared by anionic polymerization, such as those of styrene, isoprene and butadiene, that are activated for nucleophilic reactions in hydrocarbon media by conducting an ion exchange reaction between alkali metal polymeric carboxylates and salts of specific Lewis acids and bases such as tetrabutylammonium fluoride.
2. Background Information
Alkali metal compound initiated, anionic polymerization is a well-known synthetic method of preparing polymers, of such monomers as styrene, isoprene and butadiene, by which the major variables affecting polymer properties can be controlled. Under the preferred operating conditions generally known in the art, spontaneous termination or chain-transfer reactions of the forming polymer can be essentially avoided. This characteristic has led to standard use of the terms "living polymerization"or "living polymers" for such polymerization reactions and the corresponding polymers. Living polymerization allows the preparation of polymers of pre-determined molecular weight, narrow molecular weight distribution ("MWD"), and chain-end functionality. A vast array of synthetic procedures and novel polymers have resulted from the use of these well-characterized, functionalized polymers in grafting, copolymerization, and linking reactions. Specifically, the carbonation of living polymeric anions using carbon dioxide is both known and in wide use.
In "Reaction of Polystyryllithium with Carbon Dioxide", Wymanet al, Journal of Polymer Science; Part A, Vol. 2, pages 4545-4550 (1964), reported that polystyryllithium terminated with gaseous carbon dioxide yielded the polystyrenecarboxylic acid but also di-polystyryl ketone and branched tri-polystyryl carbinol in a 60/28/12 % yield, respectively. Mansson in "Reactive of Polystyryl Anions with Carbon Dioxide and Oxygen", Journal of Polymer Science, Polymer Chemistry Edition, Vol. 18, pages 1945-1956 (1980) reported yields of polystyrenecarboxylic acid lower than that reported by Wyman, et al, when the reaction of gaseous carbon dioxide and polystyryllithium at about 10.degree. C. was conducted in a solvent of mixed methylcyclohexane and tetrahydrofuran (THF), as compared to the benzene solvent of Wyman.
Young, et al., in "Advances in Polymer Science" #56, pages 70-72, noted that use of Lewis bases such as tetrahydrofuran ("THF") served to promote disaggregation of polymeric organolithium species and thus in the presence of an excess of THF, in a 75/25 mixture by volume of benzene and THF, carbonation of poly(styryl) lithium, poly(isoprenyl)lithium, and poly(styrene-b-isoprenyl)lithium, reportedly resulted in quantitative suppression of coupling side reactions. Similar results are reported by Quirk, et al., in "Functionalization of Polymeric Organolithium Compound Carbonation", Makromolecular Chemistry, 183, 2071-2076 (1982). One hundred percent yields were reported for 75/25 benzene/THF carbonation solvents. Remarks as to the expressed need for contaminate-free conditions were later discounted in Quirk and Yin, "Functionalization Reactions of Poly(styryl)lithium with Carbon Dioxide", Polymer Preprints 29, 401-402 (1987). Carboxylation yields for the earlier report are here characterized as having been "essentially quantitative." Both reports teach polymerization at 30.degree. C. under high vacuum conditions with gaseous CO.sub.2 introduced after addition of THF into the polymerization reaction vessel. The essentially quantitative yields were said to be obtained from freeze-dried solutions of poly(styryl)lithium.
The polymeric alkali metal carboxylates formed using the above techniques have limited utility for nucleophilic reactions in hydrocarbon media: the bond formed between the carboxylate anion and the preferred alkali metal cation is strong and preferred over those formed in nucleophilic reactions on carbon electrophiles. Thus it is necessary to transform the alkali metal carboxylate into another salt in order to increase the utility of these polymers. It is well known in the art (J. March, Advanced Organic Chemistry 3rd Edition, page 353, Wiley, N.Y. 1985) that replacement of lithium with sodium, potassium, or cesium increases the nucleophilicity of the carboxylate in polar non-protic solvents such as hexamethylphosphorictriamide (HMPA). Potassium carboxylates are good nucleophiles in hydrocarbon media if a complexing agent is added like 18-Crown-6. Most organic cations, which include alkyl and aryl substituted ammonium and phosphonium cations, are soluble in hydrocarbon liquids and hence polymeric ammonium carboxylates are good nucleophiles in hydrocarbon media. Thus typically prior to the disclosure of this invention, polymeric alkali metal carboxylates have been converted to the polymeric carboxylic acid form by treatment with a strong acid and then neutralized with a base having the desired cation.
However, the polymeric alkali metal carboxylates have limited stability; they are capable of undergoing decarboxylation reactions. The decarboxylation reaction occurs to a limited extent during isolation of the dry alkali metal carboxylate polymer; however, it has recently been unexpectedly observed that polymeric decarboxylation reactions are greatly accelerated upon formation of the carboxylic acid for subsequent reneutralization with a desired base. It is believed that this labile decarboxylation is the result of the placement of an activated hydrogen substituent on the carbon adjacent to both the carboxylic acid and a resonance stabilizing group, such as aryl or vinyl unsaturation, constituted by the remaining polymer. It has been surprisingly discovered that these decarboxylation reactions can be essentially eliminated, thus substantially increasing the resulting polymeric carboxylate salts available for subsequent reaction, by conducting an ion- exchange reaction without forming the polymeric carboxylic acid and subsequently neutralizing. It is believed that by eliminating the formation of the carboxylic acid, the hydrogen substituent on the polymer segment adjacent to the carbonyl grouping is not activated and thus decarboxylation is substantially depressed.
In view of the many uses for living polymers, particularly those comprised of styrene, isoprene and butadiene, having reactive end-groups capable of subsequent coupling, crosslinking, etc., the need for efficient, cost-effective means of preparation of such polymers is evident. It is thus an object of this invention to provide an improved method of functionalizing polymers produced by living polymerization whereby improved utilization of reactants is achieved while simultaneously minimizing the occurrence of undesirable side reactions and by-products.